The present invention relates generally to the field of geophysical exploration and more particularly to a novel method of magnetotelluric exploration.
Magnetotelluric exploration involves simultaneously measuring and recording the earth's magnetic field and electric field, at the earth's surface, to obtain estimates of the earth's resistivity structure. Historically, magnetotelluric exploration has involved measuring orthogonal components of the earth's magnetic and electric fields at one or more discrete locations as envisioned by Cagnaird in "Basic theory of the magnetotelluric method of geophysical prospecting" Geophysics Vol. 18, p. 605 (1953). As a result of such magnetotelluric surveys, one was able to generate one-dimensional estimates of the earth's resistivity structure for simple plane-layered earth models. Although such magnetotelluric methods required the measurement of both the electric and magnetic fields at each sensing location, it was recognized by those skilled in the art that the earth's magnetic field varies more slowly as a function of spatial location than does the earth's electric field. Consequently, closely spaced magnetotelluric methods often measure the earth's magnetic field less frequently spatially, than the earth's electric field. Present magnetotelluric exploration methods, provide techniques for obtaining two-dimensional and three-dimensional (under limited circumstances), estimates of the earth's resistivity structure by measuring two orthogonal components of the earth's magnetic and electric fields. Nevertheless, present magnetotelluric exploration methods remain discrete surveys of the earth's resistivity structure. That is, electric dipole measurements of the earth's electric field are typically obtained over specified, noncontinuous intervals which are too far apart to avoid spatial aliasing.
In an attempt to overcome the spatial aliasing of present magnetotelluric exploration methods, Bostick described in "Electromagnetic Array Profiling," 50th Annual Meeting, Society of Exploration Geophysicists, pages 60-61, (1986), an electromagnetic profiling (EMAP) method whereby continuous, rather than discrete, electric dipole measurements of one component the earth's electric field are made along a substantially straight line of profile and two orthogonal components of the earth's magnetic field are measured at at least one location in the vicinity of the line of profile. Additionally, the EMAP line of profile is aligned generally perpendicular to an assumed strike direction of the earth's formations. As used in the geophysical art, strike is the direction along which the earth's resistivity is generally constant.
Since the EMAP method obtains electric dipole measurements of only one component of the earth's electric field over specified, continuous intervals, the EMAP method can yield only one-dimensional estimates of the earth's resistivity structure. By obtaining a plurality of such one-dimensional estimates along the line of profile and placing them adjacent one another, however, the EMAP method can emulate a two-dimensional profile of the earth's resistivity structure along the line of profile. Unfortunately, if the assumption about the strike direction of the earth's formations is incorrect, the EMAP data collected can be confused and insufficient to make revised estimates of the strike direction or to obtain estimates of the earth's resistivity structure.
Both magnetotelluric and EMAP exploration techniques necessarily involve simultaneously measuring and recording one or more components of the earth's electric and magnetic fields for long periods of time, typically 24 hours, and over extended distances. The deployment of sensors, recording equipment, and their associated cables comprises a substantial portion of the cost of such exploration techniques. Magnetotelluric and EMAP methods of exploration are generally intended to provide a preliminary interpretation of the earth's substructure over the widest area possible. As such, the cost associated with collecting the most information possible over the widest area can be a significant factor to consider when selecting either magnetotelluric or EMAP exploration methods.
The EMAP exploration technique does reduce the number of recording channels (and hence cost) over present magnetotelluric exploration techniques since only one electric dipole component of the earth's electric field is measured and recorded at each sensing location along a line of profile. However, the EMAP technique does increase the length of deployed cable since it obtains continuous, rather than discontinuous, electric dipole measurements of the earth's electric field.
As a consequence of measuring only one electric dipole component of the earth's electric field along the line of profile, the EMAP technique cannot measure the complete impedance tensor along the line of profile as with the conventional magnetotelluric method of exploration nor can the EMAP method determine the strike direction of the earth's formations. Additionally, the EMAP technique cannot truly generate two-dimensional estimates of the earth's resistivity structure. Rather, the EMAP technique generates a continuous series of one-dimensional estimates of the earth's resistivity structure along the line of profile so as to emulate a two-dimensional profile of the earth's substructure. Thus, the EMAP technique barters reduced acquisition and collection costs for reduced information about the earth's resistivity structure.
When compared to the EMAP technique, the present invention provides a method for measuring the complete impedance tensor and determining formation strike direction with only a small increase in a number of recording channels and length of cable. When compared to conventional magnetotelluric techniques, the present invention provides a method of magnetotelluric acquisition which can (for an equivalent amount of data) substantially reduce the number of recording channels and thus the costs of acquisition. Moreover, the magnetotelluric data collected according to the present invention is amenable to both conventional magnetotelluric and EMAP processing techniques. Thus, the present method of magnetotelluric exploration provides a more cost effective way to obtain more complete estimates of the earth's subsurface structure.